Alkaline digestion of a mixture of cotton linters and fibrous vegetable material and product thereof

ABSTRACT

ALKALINE DIGESTION OF A MIXTURE OF COTTON LINERS, HAVING A FIBER LENGTH OF ABOUT 2 TO ABOUT 5 MILLIMETERS, AND FIBROUS VEGETABLE MATERIAL.

United States Patent 3,737,369 ALKALHWE DIGESTION OF A MIXTURE OF COT- TON LINTERS AND FIBROUS VEGETABLE MA- TERIAL AND PRODUCT THEREOF Eduardo J. Villavicencio, Paramonga, Peru, assignor to Process Evaluation and Development Corporation, New York, N.Y. No Drawing. Filed Mar. 13, 1972, Ser. No. 234,362 Int. Cl. D21c /00 US. Cl. 162-95 Claims ABSTRACT OF THE DISCLOSURE Alkaline digestion of a mixture of cotton linters, having a fiber length of about 2 to about 5 millimeters, and fibrous vegetable material.

This invention relates to methods of chemically pulping blends of diverse cellulosic materials. In particular it re lates to a process for pulping blends or mixtures of cotton linters with the fibrous fraction of vegetable materials such as straw, flax, rice hulls, hemp, esparto, bamboo and other like materials, especially depithed sugarcane bagasse. The invention further relates to the resultant pulps and to paper products prepared therefrom.

Cotton linters (the staple left on cotton seeds after ginning) have previously been used to prepare paper pulp. The linters, which are approximately 90% or more pure cellulose, are normally pulped by mechanical pulping processes using, e.g., Hollander refiners and very small amounts of reactive chemical for oil and wax removal. Pulps have also been prepared by chemical pulping of the fibrous portions of straws, grasses and other like vegetable materials such as depithed sugarcane bagasse. Bagasse and similar vegetable material fibers have a relatively very high hemicellulose content (upwards of 28% by weight of the total lignocellulose material) a substantial proportion of which is retained after a conventional soda process pulping. Because of this characteristic bagasse pulps hydrate extremely fast and are typically observed to be (1) too easily refined, and (2) intrinsically weaker, especially in regard to tearing strength of paper made therefrom. Because of the very diverse nature of cotton linters and bagasse and like vegetable fibers and the pulping processes conventionally applied to each it has not previously been considered feasible to prepare paper pulps from blends of these materials.

In accordance with the present invention it has been found that blends or mixtures of 2 to weight percent cotton linters and correspondingly 98 to 85 weight percent lignocellulosic fibers of vegetable materials such as sugarcane bagasse may be chemically pulped to produce pulps useful for the manufacture of paper. Preferably the amount of linters in the mixture is from about 5 to about 10 weight percent based on total weight. By blending the fibers before digestion xylosan (the major hemicellulose constituent of bagasse) is released from the bagasse and absorbed by the cotton linters during the alkaline digestion. The rate of hydration of the bagasse fibers is thus reduced and that of the cotton linters is increased. Pulp from the blend will take refining energy and yield paper having improved strength characteristics as compared to 100% bagasse pulp furnish.

It is an important and critical feature of the present invention that the cotton linters utilized have a maximum fiber length of not more than about 4 to 5 millimeters and a minimum fiber length of not less than about 2 to 2.5 millimeters. In trial runs of the process of this invention it has been observed that pulps including cotton linters fibers longer than the aforesaid maximum, e.g. about 6- rnillimeters cannot be satisfactorily refined; apparently be- 3,737,369 Patented June 5, 1973 cause the linters agglomerate and become mechanically entwined into relatively thick ropes which plug the refiner. When the cotton linters used are shorter than about 2 millimeters the tear strength and tensile strength properties of paper made from the resultant pulp tend to decrease as compared to paper from pulps containing cotton linters of longer fiber length. Present experience indicates that the most preferred fiber length for the cotton linters is about 3 to 4 millimeters. Overlong cotton linter fibers may be reduced in size to a suitable length for use in practicing the invention by any suitable means. One particularly suitable method that has been satisfactory in trial runs has been through the use of commercially available Sprout- Waldron (Muncy, Pa., U.S.A.) rotary knife cutters, in particular the MRP model as fully described in that companys Bulletin 261.

Except for the presence of cotton linters the pulping process of this invention is otherwise the same as typical processes used for pulping of vegetable material fibers. Thus, the Weight ratio of total lignocellulosic and cellulosic (i.e., cotton linters) materials (dry weight basis), to digestion medium is between about /5 and /3 (e.g., A); the digestion temperature is from about 340 to about 380 Fahrenheit (e.g., 350 Fahrenheit) and the digestion time ranges from about 15 minutes to several hours or more, depending primarily upon the specific lignocellulosic material being pulped. For pulping mixtures of linters and depithed sugarcane bagasse the degestion or cooking time is usually from about 15 to about 40 minutes, preferably 20 to 30 minutes. The pulping process may be combined with a preliminary acid prehydrolysis step in which case alkaline cooking times can be reduced (for example, 5 to 20 minutes, preferably 10 to 15 minutes, for bagasse-linter blends). Any desired alkaline pulping chemicals may be used. Suitable chemicals are described, for example, in Kirk-Othmers Encyclopedia of Chemical Technology, second edition, volume 16 (1968), beginning at page 702.

After digestion the pulp is refined, screened, washed, etc. in the usual manner prior to use as furnish to the paper making machine.

The invention has been specifically demonstrated for pulping of mixtures of cotton linters and sugarcane bagasse. However, it is not limited to bagasse as the lignocellulosic feed material, but is applicable as Well to mixtures of cotton linters and fibers of vegetable materials such as bamboo, various straws, hemp, sisal and the like.

In the application fo the invention to pulping of mixtures of linters and bagasse the bagasse fibers used should be as pith-free as is reasonably possible. Suitable bagasse fiber feed materials may be prepared, for example, via the use of the apparatus and/or methods described in US. Pat. 3,537,142, issued Nov. 3, 1970 or in my prior copending US. Application Ser. No. 54,580, filed July 13, 1970 and now US. Pat. 3,688,345. For highest yields it is preferred that the bagasse fiber feed material be one which has been subjected to a two-stage depithing operation, i.e., first dry depithed in accordance with the aforesaid US. Pat. 3,537,142 and then further wet" depithed in the presence of at least about 4.5, normally from about 5 to about 10, parts by weight of water per part by weight of fiber (bone-dry basis) in accordance with the said US. application Ser. No. 54,580; the entire disclosure of which is incorporated herein by reference.

The blend or mixture of cotton linters and the lignocellulosic fibers of vegetable material (such as bagasse) may be prepared in any suitable manner and at any suitable time prior to commencement of the alkaline digestion. In one presently preferred embodiment the blend is formed by adding a water slurry containing the appropriate desired amount of linters to the above described second stage wet depithing operation on the bagasse fibers;

times of the pulps to various Canadian Standard Freeness were determined and certain properties of papers prepared from the beat pulps were tested. For comparison purposes pulp was prepared from the same sugarcane 5 bagasse fibrous material alone, and from the cotton linters alone, in the same manner as described above and prop erties of sample papers made from these pulps were also thus, in eifect, forming the blend in the wet depither. In this way it is possible to remove some of the greases and oils in and on the cotton linters as received and thereby improve the xylosan transfer from the bagasse to the linters in the subsequent alkaline digestion.

The proportions of the alkaline, preferably sodium, pulping chemicals used are all expressed in terms of their TABLE I.PULP RAW MATERIALS Cotton determined. Results are shown in the following Tables I, II and III.

10 'llustrated by the following sodium oxide (Na O) equivalent. Preferably the total Na O content of the digestion liquor is about 12 weight percent.

Practice of the invention is 1 a. man v 4:0 12 .m n h w m ma w 4412 c t m n. %%M% an 470 12 SW 3361 have 0 B u r m mm H m I m m W W m r mn en Ed m pi my mb 0 m i l 1 0 n1 Wm oHoA 5 1 .15 6 hmPPmfl PmaS dn %SA 6 .lkT aT P P c m m r t .m a fan XSd g r an m rfll .h G H b u P V. m m d H b i I o u n w s f m m r am m n P a m m pa m m a an end Lam e m n EMU m. r .1 .mt mem ure Pr. 0 r m w .m em m o w m m m rF f hf 3 I86 t saa a bm a a a n e mPwwmP mmm A WPPP T pared by size reduction using Sprout- Model MRP with diflerent screens-Short fiber length 1.8 millimeters; short linters ngth 3 millimeters; short linters #3, inch screen, fiber length 4 millimeters.

gth of 8 millimeters. pulps were unsuccessful due Run number 10 ,percent Density (grams per Tensile (grams per Tear break Burst cubic Fold factor (meters) factor centimeter) number TABLE IL-PULPING RUNS (All digestions with 12% NaOH) Bagasse fibers, pereent Cotton linters (as receive percent Short cotton linters:

#1, perc n #2 #3 percent.. d, percent- Basis weight (ovendl'y, grams per meter) 1 The short cotton linters were pre nters #1, He inch screen 2, inch screen, fiber le Nora-The linters as received had a fiber len Attempts to use these in mixed material lViValdron rotary knife cutter mute.

. to serious consistent plugging of the refiner. 1n the aforementioned U.S. Pat. 3,537,142. All runs were made using a soda process cooking solution containing 12 wei ht percent (as Na O) sodium hydroxide. All digestions Feed materials:

, and washed 53%! i r b the usual manner. Thereafter, the beating or refining p mm TABLE Ill-PHYSICAL PROPERTIES OF PAPER SAMPLES Canadian Standard Freeness (minutes) tive rim:

strength of the paper samples is the tensile breaking length, in meters, determined on an Instron tensile tester with a specimen l-inch wide, 4-inches long (between test 20 jaws) and at a jaw separation rate of l-inch per 111 In the following examples the lignocellulosic material to be pulped was depithed sugarcane bagasse prepared in the apparatus and with the method described (cooks) were conducted at a ratio of about 4 parts by weight cooking liquor for each part by weight (dry weight basis) of the mixture of cotton linters and bagasse fibers 30 or of these individual materials above in the comparative runs. The digestions were each performed for 30 minutes in a closed vessel at a steam pressure of to pounds per square inch gauge. After blowdown from the digester the pulps were cleaned, centrifuged in Pulp from comparamaa Pulp from Example:

maao maao 5 Graphical plotting of the data shown in Table HI above indicates that the pulps and paper samples have the representative properties shown in the following Table IV at equivalent Canadian Standard Freeness.

TABLE IV.COMPARISON AT EQUIVALENT CANADIAN STANDARD FREENESS At Canadian Standard Examples Comparative runs Freeness of 650 milliliters 1 2 3 A B C Beating time 25.0 28.0 25.0 22. 80. 7 90.0 Tear factor-.. 63. 8 70.0 65.0 61. 300.0 119. 4 Tensile break- 5,100 5, 200 4, 100 5, 550 3, 600 2, 510 Burst factor--- 24. 0 29. 5 .5 5 22.0 10.5

At Canadian Standard Freeness of 500 Milliliters Beating time 36. 7 38. 6 36. 5 32. 0 120.0 120.0 Tear factor 59. 0 66. 0 58.0 53. 6 259.6 94. 6 Tensile break 5, 900 5, 750 4, 850 6, 030 4, 290 3, 005 Burst factor 28. 5 33. 5 25. 0 32.0 31. 0 15.0

As seen from the foregoing examples the method of this invention provides a practical procedure for pulping a blend of diverse materials to yield pulps having a good combination of properties compared to pulps prepared from vegetable material fibers alone, e.g., bagasse fibers. In all examples, the beating times to equivalent Canadian Standard Freeness are greater than in Comparative Run A and tear strengths are increased. The overall physical properties of papers prepared from the pulps of all examples, especially Example 2, are good to excellent.

I claim:

1. Method for the production of pulp comprising digesting a mixture of 2 to 15 weight percent cotton linters having a fiber length of about 2 to about 5 millimeters and 98 to 85 weight percent fibrous vegetable material with an aqueous alkaline digestion solution and recovering the resultant cellulosic pulp.

2. Method as defined in claim 1 in which the cotton linters constitute from 5 to 10 weight percent of the mixture to be pulped.

3. Method of claim 2 in which the fiber length of the cotton linter is about 3 to 4 millimeters.

4. Method of claim 3 in which the digestion is conducted at steam gauge pressures of from about to about pounds per square inch for a period of from about 15 to about 40 minutes using a 12 percent aqueous solution of sodium hydroxide as the digestion solution.

5. Method of claim 1 in which the fibrous vegetable material is depithed sugarcane bagasse.

6. Chemical pulp produced by the method of claim 5.

7. In the process of chemically pulping fibrous vegetable materials with an alkaline digestion solution the improvement comprising mixing the said material with 2 to 15 weight percent of cotton linters having a fiber length of from about 2 to about 5 millimeters thereby increasing the pulp refining time and tear strength.

8. Process as defined in claim 7 wherein the cotton linters have a fiber length of about 3 to 4 millimeters.

9. Process as defined in claim 8 wherein the cotton linters are admixed in an amount of from about 5 to about 10 weight percent.

10. Paper produced from the chemical pulp of claim 6.

References Cited UNITED STATES PATENTS 9/1921 Clapp 16295 X 1/1932 Rose et a1. 16Z95 X US. Cl. X.R. 162-96 

